A liquid crystal display module needs a backlight module to emit light to a liquid crystal display panel because the liquid crystal screen cannot light up itself. The backlight module has two types, a straight type and a lateral type which is especially necessary to a device needing two displays (such as that in a mobile phone) so as to reduce the thickness itself.
With reference to FIG. 1, a prior art backlight module having two lighting sides has a light source 90 and a light guide plate 91. The light guide plate 91 is adjacent to the light source 90 for guiding light of the light source 90 to a main display screen 92 and a sub-display screen 93 at the front and rear sides of the light guide plate 91.
A lower diffusion sheet 94, a light enhancing sheet 95 and an upper diffusion sheet 96, etc. are installed between the light guide plate 91 and the main display screen 92. The function of the lower diffusion sheet 94 is to diffuse the light to adjust the uniformity of the light to avoid reflection light spots on the main display screen 92. The light enhancing sheet 95 is used to correct the light path so as to focus the light to has a preferred directivity. Thereby, the light in the front side of the main display screen 92 is enhanced. The upper diffusion sheet 96 is not a necessary structure. The function of the upper diffusion sheet 96 is to avoid Newton rings to appear and to protect the light enhancing sheet 95. The upper diffusion sheet 96 can be replaced by a reflected light polarizing sheet so that the main display screen 92 has a preferred display effect. A reflective sheet 97, a lower diffusion sheet 94 and a light enhancing sheet 95 are sequentially installed between the light guide plate 91 and the sub-display screen 93. The reflective sheet 97 is installed between the light guide plate 91 and the sub-display screen 93 so as to reflect part of the light from the light source 90 into the main display screen 92. Since in general, the main display screen 92 is larger than the sub-display screen 93, the reflective sheet 97 is used to adjust the light of the main display screen 93 and sub-display screen 93.
However, in this prior art, part of the light is reflected by the reflective sheet 97 to the main display screen 92 so that the light to the sub-display screen 93 is insufficient. If the light source 90 is increased to increase the light to the main display screen 92 and sub-display screen 93, the cost will increase and power loss is also increased.